An inverter is an electrical device enabling generation of a voltage with a variable frequency. Inverters may be used in connection with motors for driving them with a variable frequency. An inverter may also be a part of a frequency converter used for driving a motor or another load, for example. A frequency converter typically consists of a rectifier and an inverter, between which there is a direct-voltage intermediate circuit. The rectifier and the inverter may also be positioned physically apart from each other, and one rectifier may feed several inverters via a common intermediate circuit or, alternatively, several rectifiers may feed one inverter. The intermediate circuit typically comprises a capacitance consisting of one or more capacitors.
FIG. 1 shows an example of a converter system. The exemplary converter system may comprise a rectifier 30 and an inverter 20, between which there is provided a DC intermediate circuit 10 with capacitance C. An example of the rectifier 30 is a diode bridge D1 to D6, which obtains its supply from an AC (Alternating Current) source U1, V1, W1, which is for instance a 50 or 60-Hz AC network, and an example of the inverter 20 is a semiconductor bridge implemented by means of transistors S1 to S6, such as IGBTs (Insulated-Gate Bipolar Transistor) or FETs (Field-Effect Transistor), or other controllable semiconductor switches, which may be controlled according to a modulation scheme used. The inverter 20 may be used to control the power transferred from the intermediate circuit 10 to its output U2, V2, W2. Accordingly the supply from the output U2, V2, W2 of the inverter 20 may be a three-phase AC output.
The capacitance C of the intermediate circuit 10 is connected to the semiconductor switches S1 to S6 of the inverter 20 via at least two DC conductors 100, 200. In the case of a two-level inverter as shown in the example of FIG. 1, such conductors may comprise at least one positive DC conductor 100 and one negative DC conductor 200 such that the positive DC conductor 100 is connected to a positive pole or poles of the capacitance C and the negative DC conductor 200 is connected to a negative pole or poles of the capacitance C.
A problem related to the above converter system is that in use an undesirable high frequency voltage oscillation may be generated into the DC conductors 100, 200 as a result of the operation of the converter system. In the case of DC conductors made of copper, the voltage oscillation can be reduced by increasing the length of the DC conductors and thus the resistance thereof. However, increasing the length of the DC conductors also increases the inductance thereof, which is not desirable.